The present invention relates to a method of preparing malonic acid dialkyl esters by the reaction of halogen acetic acid alkyl esters with carbon monoxide and alkali metal alcoholates or alkaline earth metal alcoholates in the presence of a catalyst containing a cobalt compound and of an alcohol on which the alcoholate is based as a solvent.
Alkali metal hydroxides dissolved in the alcohols can be used instead of the alcoholates.
A known method for preparing malonic acid dialkyl esters consists in the reaction of chloroacetic acid with alkali cyanide to produce cyanacetic acid, saponification of the nitrile, and then esterification to the diester (Ullmann 1960, vol. 12, p. 192). What is unsatisfactory about this method is the plurality of steps involved in the reaction, and the use of cyanides whose toxicity creates problems not only in handling them but also in getting rid of their residues without contaminating the environment.
In addition, malonic acid dialkyl ester can be obtained by setting out from chloroacetic acid ester with stoichiometric amounts of Na[Co(CO).sub.4 ] with the formation of cobalt carbonyl hydride as an intermediate, and the decomposition of the latter (R. F. Heck and D. S. Breslow, J. Amer. Chem. Soc., 85, 2779-82 (1963)). The yield in this reaction, however, is low.
In a number of publications the carboxylation of acetic acid alkyl esters in the presence of phenolates in aprotic solvents is also described (G. Bottacio, G. P. Chiusoli and M. G. Felicioli, Gazz. Chim. Ital. 103, 105 (1973) and German Offenlegungsschrift No. 2,038,725). However, unsatisfactory yields and the complicated processing of the reaction product are obstacles to the technical application of these reactions. Herein phenolate can be absent. Aprotic solvents can also be absent.
Another known method is based on the carbonylation of chloroacetic acid alkyl esters with carbon monoxide in an alcohol in the presence of Co.sub.2 (CO).sub.8 or iron carbonyl and basic substances such as carbonates, bicarbonates, acetates and secondary and tertiary phosphates of the alkali metals as well as oxides of the alkaline earth metals such as MgO and CaO or tertiary amines, on the condition that, with these compounds, there will be no further reaction between the malonic ester that forms and the halogen acetic esters, but this is accomplished only in the case of weakly basic substances. Disadvantages of this process consist in the fact that water is formed in the reaction, which permits the alcohol to be reused only after removal of the water, in the very high catalyst concentration and the high pressures, and, if carbonates or bicarbonates are used, in the formation of carbon dioxide which reduces the partial pressure of the carbon monoxide used and therefore requires apparatus that can withstand high pressures. See U.S. application Ser. No. 527,798, filed Nov. 27, 1974 (Group Art Unit 127). Herein water and the weakly basic substances can be absent.